Plastic material pallet

ABSTRACT

Plastic pallet comprising at least one single-piece platform obtained by extrusion and resting on spacers, the platform of which is a hollow profile consisting of two substantially flat parallel elements connected by partitions .

[0001] The present invention relates to a plastic pallet and its method of manufacture. The present invention also relates to a die for extruding profiles and/or spacers forming the pallet according to the present invention.

[0002] The specification for pallets for holding, transporting and storing static loads often specifies properties such as: durability, recyclability, stackability, compliance with mechanical impact and flexural strength standards (ISO 8611 and ISO/TR 10233 standards), extended use temperature range (often from −30 to 40° C.), low weight (most often 25 kg max), ease of cleaning, absence of pockets for bacterial growth, etc.

[0003] The use of plastic pallets has been known for a long time. Compared to their wooden equivalents, they especially have the advantage of durability and recyclability.

[0004] However, for mainly economic reasons, wooden pallets are still the most widespread on the market. However, they have many drawbacks, such as their lack of hygiene, their sensitivity to moisture, mold and to fungi, the presence of splinters, the difficulty of recycling them once they are damaged, etc.

[0005] With regard to low-load, and therefore small, pallets, plastic pallets obtained by compact injection molding (made of a single part) present a beneficial alternative to the wooden pallets since, unlike the latter, they do not require assembly operation(s) which are costly in terms of labor.

[0006] In contrast, with regard to heavy-load pallets (for example one metric ton and more), the design of a compact injection mold is so complicated (since in order to compensate for the low flexural modulus of the resins for injection, it is necessary to provide a complex network of strengthening partitions) that compact injection-molded pallets are less beneficial than pallets obtained by assembling parts. Furthermore, plastics for injection generally have insufficient mechanical properties and therefore require the use of greater thicknesses than plastics for extrusion.

[0007] With regard to plastic pallets obtained by assembling extruded parts, a known technique which is described in patent BE 848554 consists in extruding hollowed profiles in the form of strips and longitudinal members or feet, and assembling them in order to form a pallet. By suitable choice of the cross sections of the hollow profiles, these pallets may have a mechanical strength close to that of wooden pallets, with an acceptable weight and better longevity. However, these pallets have the drawback of being obtained by assembling a large number of parts, which is uneconomic and increases the number of connections and therefore the possibilities of weak points in the structure and of pockets for bacterial growth.

[0008] Plastic pallets with a single-piece platform that is possibly extruded have been proposed, especially in documents EP 696989 and WO 95/21093. However, the one-piece platforms proposed in these documents are solid plates, the simple geometry of which has not been optimized. They therefore have the drawback of increasing the weight of the pallet, of reducing its ease of handling and of increasing its unit cost.

[0009] The subject of the present invention is therefore a plastic pallet which has, compared with this solution, the advantage, for equal mechanical properties, both of low weight and of being obtained by assembly of a small number of parts.

[0010] Consequently, the present invention relates to a plastic pallet comprising at least one single-piece platform obtained by extrusion and resting on spacers, said platform being a hollow profile consisting of two substantially flat parallel elements connected by partitions.

[0011] An important aspect of the present invention lies in optimizing the geometry of the pallet (shape and thickness of the flat elements, of the partitions, of the spacers, etc) so as to obtain a high mechanical strength with low weight. More particularly, with regard to the partitions, the latter may be oblique or transverse, of any shape and/or drawn within the profile, of any shape: rectangular, square, V-shaped, W-shaped, N-shaped, cross (X)-shaped, etc. Cross-shaped partitions are most particularly suitable. These partitions may be placed in a single layer, or in several layers separated by one or more longitudinal elements. The thickness of these partitions may or may not be uniform through a transverse cross section of the platform. In particular, it may vary through a transverse cross section from one partition to another and/or within the same partition,. Advantageously, the thickness of the partitions is greater directly in line with the spacers, where the flexural stress of the loaded pallet is greatest.

[0012] Strengthening bars may also be slid inside some (or even all) partitions in order to increase the flexural strength of the pallet. These bars may be made of wood, of glass fibers impregnated with a thermoplastic or thermosetting resin, or of metal (for example aluminum or steel) . Bars made of glass fibers impregnated with resin are most particularly suitable. For reasons of ease, the strengthening bars may also be placed between the platform and the spacers; in this case, they are generally fastened to the. spacers either by adhesive bonding, or by mechanical assembly (by means of screws, rivets, bolts, etc.).

[0013] The outer surface of the profile, which consists of one of the substantially flat elements, may be smooth. It may also have hollows, grooves, protuberances or any other relief favoring stacking, ventilation, drainage, fastening, etc.

[0014] The spacers according to the present invention are profiles of any shape, solid or hollow, preferably hollow. In the case of hollow profiles, the latter may have partitions. They may be profiles of square or rectangular cross section placed with their generatrix parallel to the pallet platform, in the form of longitudinal members. These generatrices may then be either parallel, or perpendicular to the extrusion direction of the platform. Preferably, they are parallel to this direction. It may also involve profiles of any cross section, in particular round or square, placed with their generatrix perpendicular to the platform of the pallet, in the form of pads. The platform of the pallet according to the present invention is advantageously supported either by three longitudinal members which extend over the entire length of the pallet, or by three series of pads, generally three in number (that is 9 pads in total) placed instead of the longitudinal members. However, the separating force which acts on these pads under flexural stress is such that it is preferable to hold these pads either by skis (profiles binding them to one another from below), or by a second platform, possibly similar to the platform which supports the load.

[0015] Another advantageous variant consists in choosing, as pads serving as spacers, hollow profiles such as portions of extruded tubular profiles. This is because such profiles are widely available at low cost on the market. To benefit from a sufficient contact surface between these profiles and the platform of the pallet and/or the surface on which the pallet is placed, it may prove beneficial to supplement these profiles by means of one or more parts (injection-molded or extruded) that can be assembled (for example by adhesive bonding) thereon.

[0016] According to a preferred variant, the pallet according to the present invention comprises at least two platforms, which are either separated by the spacers or located on the same side thereof. In this case, both platforms are advantageously placed with their extrusion directions perpendicular to each other (that is to say that they are crossed). This last variant gives good results, especially in bending. Pallets in which two crossed platforms are superimposed contiguously and intended to support the loads directly are particularly advantageous.

[0017] All plastic types may be suitable for the pallet of the present invention. However, the plastic forming the hollow profile must be an extrudable thermoplastic which may or may not be filled. It may involve a polyolefin (polyethylene (PE) or polypropylene (PP) homopolymer or copolymer) or a polyvinyl chloride (PVC) polymer. Preferably, the plastic is PVC. In this case, the formulation of the resin (various additives such as fillers, lubricants, strengtheners, etc.) is preferably adapted so as to comply with the specification of the application, especially in terms of cold impact strength. Advantageously, the plastic may also be a resin filled with any filler in order to comply with the specification of the application. This filler may be particulate or fibrous, natural or synthetic (for example, glass fibers or balls). Natural cellulose fibers such as wood fibers are very suitable. The advantage of choosing a resin filled with natural fibers compared with a pure resin lies in markedly easier sizing of the profile during the extrusion. A polyolefin filled with wood fibers is particularly advantageous from this point of view. On the other hand, PVC has the advantage of better fire and flow resistance. It should be noted that for the purpose of decreasing the price of the pallet according to the present invention, the plastic may be a partly or completely recycled plastic.

[0018] The spacers according to the present invention are preferably also based on an extrudable plastic (which may or may not be filled). This plastic may be the same as that forming the platform(s), or different. Advantageously, the spacers are made with the same plastic as the platform(s). Preferably, all of the plastic(s) used for the pallet according to the present invention is recycled plastic. Alternatively, the spacers according to the present invention may be based on a material other than a plastic, for example cardboard, wood or metal. Wood gives good results. When the pallet according to the present invention has skis (as defined above), the latter may also be made of plastic, or cardboard, metal or wood.

[0019] The subject of the present invention is also a method of manufacturing a pallet as described above, by extrusion of hollow profiles forming the platform(s) and the spacers, and by assembly thereof. This assembly may be carried out by any technique suitable for assembling plastic parts, namely adhesive bonding, welding, riveting, etc. Preferably, this assembly takes place by adhesive bonding. The adhesives used for this purpose are preferably based on a resin that can be crosslinked (for example, in ambient air, such as a polyurethane resin (PUR)) and/or a solvent (tetrahydrofuran (THF)).

[0020] One advantage of the method according to the present invention lies in its simplicity and in the small number of profiles to be extruded and assembled. Thus, for example, according to the advantageous variant which consists in choosing spacers in the form of tube portions cut up into pads, all that is required is to choose a few profile widths and a few tube diameters in order to obtain an entire range of pallets. It should be noted that if the profiles which form the platforms are open on the sides, it is advantageous also to provide closure profiles which will be clipped or welded to the open profiles. These closure profiles may be extruded or injection-molded, depending on their shape. Preferably, they are based on the same plastic as that of the open profiles. Similarly, where the spacers are tube portions, it may prove to be advantageous (as explained above) for them also to be supplemented with one or more extruded or injection-molded parts favoring the assembly and/or the support of loads.

[0021] With regard to the extrusion of hollow profiles and spacers, this is generally carried out by shaping the plastic through a die fitted with through orifices. Thus, as explained above, the platform(s) of the pallet, and possibly also the spacers thereof, have partitions which do not necessarily have a uniform wall thickness within the same transverse cross section of the profile. Consequently, the subject of the present invention is also a die for the extrusion of hollow profiles as described above and comprises channels for the formation of partitions not having a uniform thickness through the same transverse cross section in the profile, from one partition to the other and/or within the same partition.

[0022] Thus, as explained above, the pallets according to the present invention are particularly advantageous for supporting heavy loads, for example of one metric ton and more. To do this, they generally satisfy mechanical impact and flexural strength standards (ISO 8611 and ISO/TR 10233 standards) used in the field under the load of one metric ton and this is preferably done so at a low weight (most often 25 kg max). Consequently, a subject of the present invention is also a pallet as described above which complies with the requirements of the ISO/TR 10233 (1989) standard during a bending test according to the ISO 8611 (1991) standard under a static load of a minimum of one metric ton.

[0023] The present invention is illustrated in a nonlimiting manner by the examples below.

EXAMPLE 1

[0024] A bending test according to the ISO 8611 (1991) standard was simulated on hollow profiles made of PVC. Four types of partition were simulated (see FIG. 1) and the thickness of the walls of various platforms was calculated so as to obtain an identical weight for each transverse cross section of the profile.

[0025] The table below shows that the X-shaped partitions made it possible to obtain the stiffest profile while having fairly small maximum stresses. Such a profile is therefore most particularly suitable as a platform for a pallet according to the present invention. Partition Thickness Max. deflection Max. stress type (mm) (mm) (MPa) I 2.00 16 81 W 1.77 10 12 N 0.94 5 65 X 0.88 1 14

EXAMPLE 2

[0026] The case of a pallet as described in FIG. 2 and having two platforms with X-shaped partitions, crossed with respect to each other, having a total thickness of 25 mm, with dimensions of 1000 mm by 1200 mm, and assembled by 9 tube portions of length 95 mm and diameter 100 mm, was simulated. It should be noted that the profiles forming this pallet have a variable thickness of 4 mm for the tube portions, 2 mm for the flat elements of the crossed platforms, 1.5 mm for the partitions directly in line with the feet and 0.75 mm for the other partitions. Such a pallet weighs 23 kg and complies with the requirements of the ISO/TR 10233 (1989) standard during simulation of a bending test according to the ISO 8611 (1991) standard under a static load of one metric ton at 40° C. In particular, the following maximum deflections (in mm) were observed: Transverse load Longitudinal load Instantaneous 12 21 After 24 h 14 26 Max. according to 21 26 ISO/TR 10233 (1989)

[0027] Furthermore, the maximum Von Mises stresses (in Mpa) measured during a 24 h test were distributed relatively uniformly in the various elements of the pallet: Part in question Transverse load Longitudinal load Platform skins 15.2 16.6 Ribs 13.2 15.0 Feet 12.1 17.2

EXAMPLE 3

[0028] A pallet made of rigid PVC consisting of two crossed and superimposed platforms fitted with I-shaped partitions, supported by 9 tube portions modified by the addition of injection-molded parts giving them the shape of tetrahedral single-serving cartons, and connected together by 9 skis (so as to obtain a symmetrical structure), was tested in bending according to the ISO 8611 (1991) standard under a static load of one metric ton and at ambient temperature. A rear view of this pallet (that is to say of the spacers and of the skis) is shown in FIG. 3, and a profile view, in FIG. 4.

[0029] After 24 h under 1.25 metric tons (that is 1.25 times the nominal load, or maximum load), the measured deflection was 13 mm and the return (that is the deflection subsequently measured on decreasing the load to 100 kg (that is 0.1 times the nominal charge) and keeping it there for one hour) was 2 mm. The maximum values not to be exceeded according to the ISO/TR 10233 (1989) standard are respectively 24 and 10 mm for this pallet, given its dimensions.

[0030] Comparative example 4

[0031] A hollow pallet made of polyethylene marketed by WAVIN injection-molded as a single block and the platforms of which do not have substantially flat parallel elements, but only partitions describing a square mesh, was tested as in example 3. A rear view of this pallet (that is to say of the spacers and of the skis) is shown in FIG. 5, and a profile view, in FIG. 6. Values of 35 mm and 11 mm were respectively obtained for the maximum deflection and the return, the target values according. to the ISO/TR 10233 (1989) standard being 26 and 10 mm given the smaller dimensions of this pallet with respect to that of example 3. 

1. A plastic pallet comprising at least one single-piece platform obtained by extrusion and resting on spacers, characterized in that the platform is a hollow profile consisting of two substantially flat parallel elements connected by partitions.
 2. The pallet as claimed in claim 1, characterized in that the partitions are cross-shaped.
 3. The pallet as claimed in either of the preceding claims, characterized in that the thickness of the partitions is greater directly in line with the spacers.
 4. The pallet as claimed in any one of the preceding claims, characterized in that it comprises at least two platforms.
 5. The pallet as claimed in claim 4, characterized in that the two platforms are placed with their extrusion directions perpendicular to each other.
 6. The pallet as claimed in any one of the preceding claims, characterized in. that the spacers are hollow profiles.
 7. The pallet as claimed in any one of the preceding claims, characterized in that the plastic is a polyvinyl chloride or a polyolefin filled with wood fibers.
 8. The pallet as claimed in any one of the preceding claims, characterized in that it complies with the requirements of the ISO/TR 10233 (1989) standard during a bending test according to the ISO 8611 (1991) standard under a static load of a minimum of one metric ton.
 9. A method of manufacturing a pallet according to any one of the preceding claims, by extrusion of hollow profiles and by assembly thereof.
 10. A die for extruding a hollow profile comprising partitions, characterized in that it comprises channels for the formation of partitions not having a uniform wall thickness through the same transverse cross section in the profile, from one partition to another and/or within the same partition. 